Lubricating oil composition

ABSTRACT

The present invention provides a lubricating oil composition which has excellent anti-wear properties for a metal pulley and a metal belt and initial anti-shudder properties and can retain the anti-shudder properties for a long period of time, suitable for use in a continuously variable transmission with a slip-controlled wet clutch and a metal belt. The lubricating oil composition comprises a lubricating base oil, (A) an alkylsalicylic acid metal salt and/or an (overbased) basic salt thereof, wherein the component ratio of the monoalkylsalicylic acid metal salt is from 85 to 100 percent by mole, the component ratio of the dialkylsalicylic acid metal salt is from 0 to 15 percent by mole and the component ratio of the 3-alkylsalicylic acid metal salt is from 40 to 100 percent by mole; (B) a specific nitrogen compound; and (C) a phosphorus-containing anti-wear agent.

FIELD OF THE INVENTION

The present invention relates to lubricating oil compositions suitable for a transmission equipped with a slip-controlled wet clutch and more specifically to transmission lubricating oil compositions suitable for a continuously variable transmission equipped with a slip-controlled wet clutch and a metal belt, in particular a wet starting clutch, which compositions have excellent anti-wear properties for the metal pulleys and metal belt of the transmission and excellent initial anti-shudder properties and can retain the anti-shudder properties for a long period of time because the compositions contain a specific salicylate detergent.

BACKGROUND OF THE INVENTION

Recent automatic transmissions have been demanded to be light and small and sought to be improved in power transmission capability in connection with the increased power output of the engines with which the transmissions are used in combination. Therefore, lubricating oils used for such transmissions have been required to have enhanced lubricating properties, i.e., properties to prevent wear on the surfaces of bearings and gears. Metal belt type continuously variable transmissions have been also increased in torque transmitted between the metal pulleys and metal belt due to the increased power output of the engines. Therefore, the lubricating oil used for such transmissions have been required to have higher torque capacity and properties to prevent wear between the metal surfaces of the metal pulleys and metal belt.

There are some automatic transmissions or continuously variable transmissions that control the lock-up clutch built in the torque converter to be slipped at a low velocity (slip lock-up control). These transmissions have been improved with the slip lock-up control so that the drive feeling can be improved by absorbing a torque variation and the engine torque can be transmitted to the transmission mechanism efficiently. Some of the continuously variable transmissions are provided with a wet starting clutch which is initially allowed to slip and then coupled so as to start the vehicle smoothly from the halt, that is a so-called slip control. Lubricating oils used for the transmissions wherein slip control for the lock-up clutch or starting clutch is carried out are required to have excellent initial anti-shudder properties and be able to retain the anti-shudder properties for a long period of time.

Under these circumstances, transmission lubricating oil compositions are disclosed, in which a friction modifier, a metallic detergent, an ashless dispersant, and an anti-wear agent are optimally added so as to retain the friction characteristics of a lock-up clutch in a good condition and provide long-lasting initial anti-shudder properties (see Patent Documents 1 to 6 below).

For example, Patent Document 1 discloses a transmission lubricating oil composition comprising a specific calcium salicylate, an SP-based extreme pressure additive, a specific succinimide and a boron-containing ashless dispersant, each in a specific amount, which composition exhibits excellent properties such as excellent anti-shudder properties and long-lasting fatigue life. Patent Document 2 discloses a continuously variable transmission lubricating oil composition containing an organic acid metal salt with a specific structure, an anti-wear agent, and a boron-containing succinimide, as essential components, which composition has both higher friction coefficient between metals and anti-shudder properties for a slip control mechanism. Patent Document 3 discloses a long-lasting continuously variable transmission lubricating oil composition comprising calcium salicylate, a phosphorus-containing anti-wear agent, a friction modifier, and a dispersant type viscosity index improver, which composition has both a higher friction coefficient between metals and anti-shudder properties for a slip control mechanism. Patent Document 4 discloses a lubricating oil composition comprising a dithiocarbamate compound, a condensate of a branched fatty acid having 8 to 30 carbon atoms and amine, and an aminic anti-oxidant, which composition has excellent and long-lasting anti-shudder properties. Patent Document 5 discloses an automatic transmission fluid composition comprising calcium sulfonate, phosphorus acid esters and further a sarcosine derivative or a reaction product of a carboxylic acid and amine, which composition has long-lasting anti-shudder properties for a slip lock-up mechanism and long-lasting properties to prevent scratch noise in a belt type continuously variable transmission. Patent Document 6 discloses an automatic transmission fluid composition comprising a specific alkaline earth metal sulfonate in a specific amount, which composition is excellent in oxidation stability as a fluid used for an automatic transmission with a slip control mechanism and has long-lasting anti-shudder properties.

However, as the results of studies conducted by the inventors of the present invention, it was found that the use of a sulfonate detergent as a metallic detergent was still insufficient to improve initial anti-shudder properties, and in particular when a monoalkyl type salicylate detergent was used, the resulting lubricating oil composition was excellent in initial anti-shudder properties but was likely to cause wear or elution of metal ions such as iron ion in a continuously variable transmission wherein the metal pulleys contact the metal belt under severe conditions and was difficult in retaining anti-shudder properties even though blended with the friction modifiers as mentioned in the above patent documents.

Patent Document 7 describes that since the monoalkyl type salicylate detergent inhibits the anti-wear effect of a phosphorus-containing additive, the use of a specific amide compound in combination therewith can enhance anti-wear properties for the valve train of an internal combustion engine. However, Patent Document 7 does not disclose or even suggest any solution for preventing wear in a transmission with a slip controlled type wet clutch, particularly a continuously variable transmission wherein the metal pulleys contact the metal belt under severe conditions or preventing shudder peculiar to the slip controlled type wet clutch for a long period of time.

-   -   (1) Patent Document 1: Japanese Patent Laid-Open Publication No.         2003-113391     -   (2) Patent Document 2: Japanese Patent Laid-Open Publication No.         2001-323292     -   (3) Patent Document 3: Japanese Patent Laid-Open Publication No.         2000-355695     -   (4) Patent Document 4: Japanese Patent Laid-Open Publication No.         11-50077     -   (5) Patent Document 5: Japanese Patent Laid-Open Publication No.         10-306292     -   (6) Patent Document 6: Japanese Patent Laid-Open Publication No.         10-25487     -   (7) Patent Document 7: Japanese Patent Laid-Open Publication No.         2004-67812

DISCLOSURE OF THE INVENTION

The present invention has an object to provide a lubricating oil composition suitable for a continuously variable transmission with a slip-controlled wet clutch, metal pulleys and a metal belt, particularly such a transmission with a slip-controlled starting clutch, which composition has excellent anti-wear properties for the metal pulley and metal belt and initial anti-shudder properties and can retain the anti-shudder properties for a long period of time.

As the results of extensive studies conducted by the inventors, the present invention was accomplished on the basis of the finding that the above object was able to be achieved using a specific salicylate detergent, a specific nitrogen compound and a phosphorus-containing anti-wear agent in combination.

That is, according to the present invention, there is provided a lubricating oil composition suitable for a transmission with a slip-controlled wet clutch, which composition comprises a lubricating base oil, (A) an alkylsalicylic acid metal salt and/or an (overbased) basic salt thereof, wherein the component ratio of the monoalkylsalicylic acid metal salt is from 85 to 100 percent by mole, the component ratio of the dialkylsalicylic acid metal salt is from 0 to 15 percent by mole and the component ratio of the 3-alkylsalicylic acid metal salt is from 40 to 100 percent by mole, (B) a nitrogen compound represented by formula (1) below, and (C) a phosphorus-containing anti-wear agent:

wherein R¹ is a straight-chain or branched alkyl group having 1 to 30 carbon atoms, R² is hydrogen or a straight-chain or branched alkyl group having 1 to 24 carbon atoms, and a and b are each independently an integer of 1, 2 or 3.

Preferably, the lubricating oil composition further comprises a friction modifier, particularly an amine-type friction modifier and/or a fatty acid-type friction modifier.

Preferably, the lubricating oil composition further comprises calcium sulfonate and/or magnesium sulfonate.

Preferably, the lubricating oil composition further comprises a boron-free ashless dispersant and/or a boron-containing ashless dispersant.

Preferably, the alkylsalicylic acid metal salt is a secondary alkyl type alkylsalicylic acid metal salt.

Preferably, the lubricating oil composition is used for a continuously variable transmission with metal pulleys and a metal belt.

Preferably, the slip-controlled wet clutch is a slip-controlled starting clutch.

The present invention also relates to a method of lubricating a continuously variable transmission with metal pulleys and a metal belt and/or a slip-controlled wet clutch.

The present invention will be described in more detail below.

There is no particular restriction on the lubricating base oil of the lubricating oil composition of the present invention. Therefore, the lubricating base oil may be a mineral base oil or a synthetic base oil.

Specific examples of the mineral oil include those which can be produced by subjecting a lubricating oil fraction produced by vacuum-distilling an atmospheric distillation bottom oil resulting from atmospheric distillation of a crude oil, to any one or more treatments selected from solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, and hydrorefining; wax-isomerized mineral oils; and those produced by isomerizing GTL WAX (Gas to Liquid Wax).

Examples of the synthetic lubricating base oil include polybutenes and hydrogenated compounds thereof; poly-α-olefins such as 1-octene oligomer and 1-decene oligomer, and hydrogenated compounds thereof; diesters such as ditridecyl glutarate, di-2-ethylhexyl adipate, diisodecyl adipate, ditridecyl adipate and di-2-ethylhexyl sebacate; polyol esters such as neopentyl glycol ester, trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethylhexanoate and pentaerythritol pelargonate; aromatic synthetic oils such as alkylnaphthalenes, alkylbenzenes, and aromatic esters; and mixtures of the foregoing.

Examples of the lubricating base oil which may be used in the present invention include the above-described mineral base oils and synthetic base oils and mixtures of two or more oils selected from these base oils. For example, the base oil used in the present invention may be one or more of the mineral base oils or synthetic base oils or a mixed oil of one or more of the mineral base oils and one or more of the synthetic base oils.

There is no particular restriction on the kinematic viscosity of the lubricating base oil of the present invention. However, the lubricating base oil is preferably so adjusted that the kinematic viscosity at 100° C. is preferably from 2 to 8 mm²/s, more preferably from 2.5 to 6 mm²/s, particularly preferably from 3 to 4.5 mm²/s. A base oil with a kinematic viscosity at 100° C. of greater than 8 mm²/s is not preferable because the resulting lubricating oil composition would be poor in low temperature viscosity characteristics while a base oil with a kinematic viscosity at 100° C. of less than 2 mm²/s is not also preferable because the resulting lubricating oil composition would be poor in lubricity due to its insufficient oil film formation at lubricating sites and large in evaporation loss of the lubricating base oil.

There is no particular restriction on the sulfur content in the lubricating base oil. However, the sulfur content is preferably 0.1 percent by mass or less, more preferably 0.05 percent by mass or less, more preferably 0.01 percent by mass or less.

There is no particular restriction on the evaporation loss of the lubricating base oil. However, the NOACK evaporation loss is preferably from 10 to 50 percent by mass, more preferably from 20 to 40 percent by mass, particularly preferably from 22 to 35 percent by mass. The use of a lubricating base oil with a NOACK evaporation loss adjusted within the above ranges renders it possible to achieve both low temperature characteristics and anti-wear properties. The term “NOACK evaporation loss” used herein denotes an evaporation loss measured in accordance with CEC L-40-T-87.

Specifically, the lubricating base oil is preferably a mixture of a base oil with a kinematic viscosity at 100° C. of 1.5 to 3.5 mm²/s, preferably 2 to 3.2 mm²/s, more preferably 2.5 to 3 mm²/s, a sulfur content of 0.05 percent by mass or less, preferably 0.01 percent by mass or less, more preferably 0.005 percent by mass or less and a NOACK evaporation loss of 20 to 80 percent by mass, preferably 30 to 65 percent by mass, more preferably 30 to 55 percent by mass and a base oil with a kinematic viscosity at 100° C. of 3.5 to 6 mm²/s, preferably 3.8 to 4.5 mm²/s, more preferably 3.9 to 4.5 mm²/s, a sulfur content of 0.05 percent by mass or less, preferably 0.01 percent by mass or less, more preferably 0.005 percent by mass or less, and a NOACK evaporation loss of 5 to 20 percent by mass, preferably 10 to 18 percent by mass, more preferably 12 to 16 percent by mass, mixed at a mass ratio of 10:90 to 90:10, preferably 25:75 to 75:25, more preferably 40:60 to 60:40 so that the kinematic viscosity at 100° C., sulfur content and NOACK evaporation loss of the mixed base oils are within the above-described ranges. As the result, it is rendered possible to produce a composition having both low temperature characteristics and lubricating properties suitable for a transmission oil composition.

Optionally, the above-described mixed base oil may contain a base oil with a kinematic viscosity at 100° C. of 6 mm²/s or greater, preferably 10 to 35 mm²/s, a sulfur content of 0.05 to 1 percent by mass, preferably 0.1 to 0.7 percent by mass, more preferably 0.2 to 0.6 percent by mass, and a NOACK evaporation loss of 10 percent by mass or less, preferably 5 percent by mass or less, more preferably 3 percent by mass or less, in a small amount, for example 5 to 30 percent by mass.

A component specifically defined as Component (A) in the lubricating oil composition of the present invention is an alkylsalicylic acid metal salt and/or an (overbased) basic salt thereof, wherein the component ratio of the monoalkylsalicylic acid metal salt is from 85 to 100 percent by mole, the component ratio of the dialkylsalicylic acid metal salt is from 0 to 15 percent by mole and the component ratio of the 3-alkylsalicylic acid metal salt is from 40 to 100 percent by mole.

The term “monoalkylsalicylic acid metal salt” used herein denotes an alkylsalicylic acid metal salt having one alkyl group, such as a 3-alkylsalicylic acid metal salt, a 4-alkylsalicylic acid metal salt, and a 5-alkylsalicylic acid metal salt. The component ratio of the monoalkylsalicylic acid metal salt is from 85 to 100 percent by mole, preferably from 88 to 98 percent by mole, more preferably from 90 to 95 percent by mole, on the basis of 100 percent by mole of the alkylsalicylic acid metal salt. The component ratio of the alkylsalicylic acid metal salt other than the monoalkylsalicylic acid metal salt, such as dialkylsalicylic acid metal salts is from 0 to 15 percent by mole, preferably from 2 to 12 percent by mole, more preferably from 5 to 10 percent by mole.

The component ratio of the 3-alkylsalicylic acid metal salt is from 40 to 100 percent by mole, preferably from 45 to 80 percent by mole, more preferably from 50 to 60 percent by mole, on the basis of 100 percent by mole of the alkylsalicylic acid metal salt. The total component ratio of the 4-alkylsalicyclic acid metal salt and 5-alkylsalicylic acid metal salt corresponds to the component ratio of the alkylsalicylic acid metal salt excluding the 3-alkylsalicylic acid metal salt and dialkylsalicylic acid metal salt and is from 0 to 60 percent by mole, preferably from 20 to 50 percent by mole, more preferably from 30 to 45 percent by mole, on the basis of 100 percent by mole of the alkylsalicylic acid metal salt. Inclusion of a slight amount of a dialkylsalicylic acid metal salt renders it possible to produce a composition having both anti-wear properties and low temperature characteristics. The component ratio of the 3-alkylsalicylate of 40 percent by mole or more renders it possible to reduce relatively the component ratio of the 5-alkylsalicylic acid metal salt and thus enhance the oil solubility of the resulting composition.

Examples of the alkyl group of the alkylsalicylic acid metal salt constituting Component (A) include alkyl groups having 10 to 40, preferably 10 to 19 or 20 to 30, more preferably 14 to 18 or 20 to 26, particularly preferably 14 to 18 carbon atoms. Examples of alkyl groups having 10 to 40 carbon atoms include those such as decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl, and triacontyl groups. These alkyl groups may be straight-chain or branched and primary and secondary alkyl groups. However, secondary alkyl groups are preferable because a salicylic acid metal salt satisfying the above-described requirements for Component (A) can be produced easily.

Examples of the metal of the alkylsalicylic acid metal salt include alkali metals such as sodium and potassium, and alkaline earth metals such as calcium and magnesium. The metal is preferably calcium or magnesium, particularly preferably calcium.

There is no particular restriction on the method of producing Component (A) used in the present invention which thus may be produced by any of the known methods. For example, an alkylsalicylic acid containing a monoalkylsalicylic acid as the main component is produced by alkylating 1 mole of a phenol using 1 mole or more of an olefin having 10 to 40 carbon atoms, such as a polymer or copolymer of ethylene, propylene, or butene, preferably a straight-chain α-olefin such as an ethylene polymer, and then carboxylating the alkylated phenol using carbon dioxide gas, or alternatively by alkylating 1 mole of salicylic acid using 1 mole or more of such an olefin preferably such a straight-chain α-olefin. The alkylsalicylic acid is then reacted with a metal base such as an alkali metal or alkaline earth metal oxide or hydroxide or converted to an alkali metal salt such as sodium salt or potassium salt, which alkali metal salt may be further substituted with an alkaline earth metal. Particularly preferably, the reaction ratio of the phenol or salicylic acid to the olefin is adjusted to preferably 1:1 to 1.15 (molar ratio), more preferably 1:1.05 to 1.1 (molar ratio) because the component ratio of the monoalkylsalicylic acid metal salt to dialkylsalicylic acid metal salt is easily adjusted to the desired ratio required for component (A) specified in the present invention. Further, particularly preferably a straight-chain α-olefin is used as the olefin because the component ratio of the 3-alkylsalicylic acid metal salt, 5-alkylsalicylic acid metal salt, or the like is easily adjusted to the desired ratio as required for Component (A), and an alkylsalicylic acid metal salt having a secondary alkyl group which is preferable in the present invention can be obtained as the main component. The use of a branched olefin as the above-mentioned olefin is not preferable because only the 5-alkylsalicylic acid metal salt is easily produced, but it is necessary to improve the oil solubility by mixing the 3-alkylsalicylic acid metal salt so as to produce Component (A) with the structure specified by the present invention, making the process variable.

Component (A) used in the present invention also includes basic salts produced by heating an alkali metal or alkaline earth metal salicylate (neutral salt) produced as described above, together with an excess amount of an alkali metal or alkaline earth metal salt or an alkali metal or alkaline earth metal base (hydroxide or oxide of an alkali metal or alkaline earth metal) in the presence of water; and overbased salts produced by reacting such a neutral salt with a base such as a hydroxide of an alkali metal or alkaline earth metal in the presence of carbonic acid gas and/or boric acid or borate.

These reactions are generally carried out in a solvent (aliphatic hydrocarbon solvents such as hexane, aromatic hydrocarbon solvents such as xylene, and light lubricating base oil). It is preferred to use a solvent whose metal content is within the range of 1.0 to 20 percent by mass, preferably 2.0 to 16 percent by mass.

Most preferred for Component (A) used in the present invention are alkylsalicylic acid metal salts and/or (overbased) basic salts thereof, the component ratios of which monoalkylsalicylic acid metal salt and dialkylsalicylic acid metal salt are from 85 to 95 percent by mole and from 5 to 15 percent by mole respectively, and 3-alkylsalicylic acid metal salt, and both 4-alkylsalicylic acid metal salt and 5-alkylsalicylic acid metal salt are from 50 to 60 percent by mole and from 35 to 45 percent by mole respectively, because the resulting lubricating oil composition will be excellent in initial anti-shudder properties. The alkyl group referred herein is particularly preferably a secondary alkyl group.

The base number of Component (A) used in the present invention is usually from 0 to 500 mgKOH/g, preferably from 20 to 450 mgKOH/g, particularly preferably from 100 to 300 mgKOH/g. One or more compound with a base number in these ranges may be used. The term “base number” used herein denotes a base number measured by the perchloric acid potentiometric titration method in accordance with section 7 of JIS K2501 “Petroleum products and lubricants-Determination of neutralization number”.

The content of Component (A) in the lubricating oil composition of the present invention is preferably from 0.005 to 0.5 percent by mass, more preferably from 0.01 to 0.2 percent by mass, more preferably from 0.02 to 0.1 percent by mass, particularly preferably from 0.02 to 0.05 percent by mass in terms of metal on the basis of the total mass of the composition in view of anti-wear properties, initial anti-shudder properties and retainability thereof. Component (A) in a relatively less amount is better.

Component (B) used in the present invention is a nitrogen compound represented by formula (1):

In formula (1), R¹ is a straight-chain or branched alkyl group having 1 to 30, preferably 6 to 24, more preferably 6 to 12 carbon atoms. R² is hydrogen or a straight-chain or branched alkyl group having 1 to 24 carbon atoms, preferably hydrogen or a straight-chain or branched alkyl group having 1 to 12 carbon atoms, more preferably a straight-chain or branched alkyl group having 6 to 12 carbon atoms. The letters “a” and “b” each denote an integer of 1, 2 or 3, preferably 1 or 2, more preferably both denote an integer of 2.

Specific examples of the straight-chain or branched alkyl group having 1 to 30 carbon atoms for R¹ include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, hexyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl, and triacontyl groups (these alkyl groups may be straight-chain or branched). Examples of the straight-chain or branched alkyl groups having 1 to 24 carbon atoms for R² include those having 1 to 24 carbon atoms, exemplified with respect to R¹.

In a nitrogen compound represented by formula (1) it is preferred that R¹ is an alkyl group having 6 to 24 carbon atoms, R² is hydrogen or an alkyl group having 1 to 24 carbon atoms, a and b are each 1 or 2 and particularly preferred that R¹ and R² are each an alkyl group having 6 to 12, more preferably 6 to 10 carbon atoms, and both a and b are 2.

Examples of preferred nitrogen compounds represented by formula (1) include 2,5-bis(alkylthio)-1,3,4-thiadiazole having a straight-chain or branched alkyl group having 6 to 24 carbon atoms; 2,5-bis(alkyldithio)-1,3,4-thiadiazole having a straight-chain or branched alkyl group having 6 to 24 carbon atoms; 2-(alkylthio)-5-mercapto-1,3,4-thiadiazole having a straight-chain or branched alkyl group having 6 to 24 carbon atoms; 2-(alkyldithio)-5-mercapto-1,3,4-thiadiazole having a straight-chain or branched alkyl group having 6 to 24 carbon atoms, and mixtures thereof. Among these, particularly preferred are 2,5-bis(alkyldithio)-1,3,4-thiadiazoles.

The content of the nitrogen compound represented by formula (1) in the lubricating oil composition of the present invention is preferably from 0.005 to 0.5 percent by mass, more preferably from 0.01 to 0.2 percent by mass, more preferably from 0.02 to 0.15 percent by mass, particularly preferably from 0.03 to 0.14 percent by mass on the basis of the total mass of the composition. The content of the nitrogen compound within the foregoing ranges makes it easy to retain anti-wear properties and anti-shudder properties for a long period of time.

Component (C) used in the present invention is a phosphorus-containing anti-wear agent.

There is no particular restriction on the phosphorus-containing anti-wear agent as long as it contains phosphorus in its molecules. Examples of the anti-wear agent include phosphorus compounds represented by formula (2) or (3) below or salts thereof:

In formulas (2) and (3), X¹, X², X³, X⁴, X⁵, and X⁶ are each independently oxygen or sulfur, and R³, R⁴, R⁵, R⁶, R⁷, and R⁸ are each independently hydrogen or a hydrocarbon group having 1 to 30 carbon atoms.

Specific examples of the hydrocarbon groups having 1 to 30 carbon atoms for R³ to R⁸ include alkyl, cycloalkyl, alkenyl, alkyl-substituted cycloalkyl, aryl, alkyl-substituted aryl, and arylalkyl groups.

Examples of the alkyl group include those, which may be straight-chain or branched, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl and octadecyl groups.

Examples of the cycloalkyl group include those having 5 to 7 carbon atoms, such as cyclopentyl, cyclohexyl and cycloheptyl groups. Examples of the alkylcycloalkyl group include those, of which the alkyl group may bond to any position of the cycloalkyl group, having 6 to 11 carbon atoms, such as methylcyclopentyl, dimethylcyclopentyl, methylethylcyclopentyl, diethylcyclopentyl, methylcyclohexyl, dimethylcyclohexyl, methylethylcyclohexyl, diethylcyclohexyl, methylcycloheptyl, dimethylcycloheptyl, methylethylcycloheptyl and diethylcycloheptyl groups.

Examples of the alkenyl group include those which may be straight-chain or branched and the position of which the double bond may vary, such as butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl, and octadecenyl groups.

Examples of the aryl group include phenyl and naphtyl groups. Examples of the alkylaryl group include those of which the alkyl group may be straight-chain or branched and bond to any position of the aryl group, having 7 to 18 carbon groups, such as tolyl, xylyl, ethylphenyl, propylphenyl, butylphenyl, pentylphenyl, hexylphenyl, heptylphenyl, octylphenyl, nonylphenyl, decylphenyl, undecylphenyl, and dodecylphenyl groups.

Examples of the arylalkyl group include those of which the alkyl group may be straight-chain or branched, having 7 to 12 carbon atoms, such as benzyl, phenylethyl, phenylpropyl, phenylbutyl, phenylpentyl and phenylhexyl groups.

The hydrocarbon group having 1 to 30 carbon atoms for R³ to R⁸ is preferably an alkyl group having 1 to 30 carbon atoms or aryl group having 6 to 24 carbon atoms, more preferably an alkyl group having 4 to 20 carbon atoms, more preferably an alkyl group having 6 to 18 carbon atoms.

Examples of phosphorus compounds represented by formula (2) include phosphorous acid; monothiophosphorus acid; dithiophosphorus acid; trithiophosphorus acid; phosphorus acid monoesters, monothiophosphorus acid monoesters, dithiophosphorus acid monoesters, and trithiophosphorus acid monoesters, each having one of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphorus acid diesters, monothiophosphorus acid diesters, dithiophosphorus acid diesters, and trithiophosphorus acid diesters, each having two of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphorus acid triesters, monothiophosphorus acid triesters, dithiophosphorus acid triesters, and trithiophosphorus acid triesters, each having three of the above-described hydrocarbon groups having 1 to 30 carbon atoms; and mixtures thereof.

Examples of phosphorus compounds represented by formula (3) include phosphoric acid; monothiophosphoric acid; dithiophosphoric acid; trithiophosphoric acid; tetrathiophosphoric acid; phosphoric acid monoesters, monothiophosphoric acid monoesters, dithiophosphoric acid monoesters, trithiophosphoric acid monoesters, and tetrathiophosphoric acid monoesters, each having one of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphoric acid diesters, monothiophosphoric acid diesters, dithiophosphoric acid diesters, trithiophosphoric acid diesters, and tetrathiophosphoric acid diesters, each having two of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphoric acid triesters, monothiophosphoric acid triesters, dithiophosphoric acid triesters, trithiophosphoric acid triesters, and tetrathiophosphoric acid triesters, each having three of the above-described hydrocarbon groups having 1 to 30 carbon atoms; and mixtures thereof.

In the present invention, preferably one or more, more preferably two or more, and most preferably all three of X¹, X², and X³ in formula (2) are oxygen.

Preferably one or more, more preferably two or more, and most preferably all three of X⁴, X⁵, and X⁶ in formula (3) are oxygen.

Examples of salts of phosphorus compounds represented by formulas (2) and (3) include salts produced by allowing a phosphorus compound to react with a nitrogen compound such as ammonia, an amine compound having in its molecules only a hydrocarbon group or a hydroxyl group-containing hydrocarbon group, having 1 to 18, preferably 1 to 8 carbon atoms, or an alkyleneoxide adduct of such an amine compound, or a metal base such as a metal oxide, a metal hydroxide, a metal carbonate and a metal chloride and neutralize the whole or part of the remaining acid hydrogen.

Specific examples of the nitrogen compound include ammonia; alkyl- or alkenyl-amines, of which the alkyl or alkenyl group may be straight-chain or branched, such as monomethylamine, monoethylamine, monopropylamine, monobutylamine, monopentylamine, monohexylamine, monoheptylamine, monooctylamine, monolaurylamine, monostearylamine, monooleylaminedimethylamine, methylethylamine, diethylamine, methylpropylamine, ethylpropylamine, dipropylamine, methylbutylamine, ethylbutylamine, propylbutylamine, dibutylamine, dipentylamine, dihexylamine, diheptylamine and dioctylamine; alkanolamines, of which the alkanol group may be straight-chain or branched, such as monomethanolamine, monoethanolamine, monopropanolamine, monobutanolamine, monopentanolamine, monohexanolamine, monoheptanolamine, monooctanolamine, monononanolamine, dimethanolamine, methanolethanolamine, diethanolamine, methanolpropanolamine, ethanolpropanolamine, dipropanolamine, methanolbutanolamine, ethanolbutanolamine, propanolbutanolamine, dibutanolamine, dipentanolamine, dihexanolamine, diheptanolamine and dioctanolamine; adducts wherein an alkyleneoxide such as ethyleneoxide is added to these alkylamines or alkanolamines; and mixtures thereof.

Specific examples of the metals of the above-mentioned metal bases include alkali metals such as lithium, sodium, potassium, and cesium, alkaline earth metals such as calcium, magnesium, and barium, and heavy metals such as zinc, copper, iron, lead, nickel, silver, manganese, and molybdenum. Among these metals, preferred are alkaline earth metals such as calcium and magnesium, and zinc.

One or more Components (C) may be arbitrary blended in the lubricating oil composition of the present invention.

Component (C) is preferably one compound or a mixture of two or more compounds, selected from phosphorous or phosphoric acid, phosphoric or phosphorus acid esters having an alkyl group having 4 to 20 carbon atoms or an (alkyl)aryl group having 6 to 12 carbon atoms and amine salts produced by allowing these esters to react with an alkylamine having an alkyl group having 1 to 18 carbon atoms, more preferably one compound or a mixture of two or more compounds, selected from phosphorus acid and phosphorus acid esters having an alkyl group having 4 to 20 carbon atoms, and particularly preferably phosphorus acid diesters having an alkyl group having 4 to 8 carbon atoms.

The content of Component (C) in the lubricating oil composition of the present invention is usually from 0.01 to 5 percent by mass on the basis of the total mass of the composition, but is preferably from 0.001 to 0.1 percent by mass, more preferably from 0.005 to 0.08 percent by mass, more preferably from 0.01 to 0.06 percent by mass, particularly preferably from 0.02 to 0.05 percent by mass in terms of phosphorus. Component (C) contained in the above ranges renders it possible to produce a composition which is excellent in anti-wear properties and initial anti-shudder properties and can easily retain the anti-shudder properties for a long period of time.

In the lubricating oil composition of the present invention, there is no particular restriction on the mass ratio ((M)/(P)) of the content of Component (A) defined by the metal element concentration (M) to the content of Component (C) defined by the phosphorus element concentration (P), on the basis of the total mass of the composition. However, the mass ratio ((M)/(P)) is preferably from 0.1 to 250, more preferably from 0.5 to 50, more preferably from 0.8 to 5, particularly preferably from 1 to 3.

In the lubricating oil composition of the present invention, there is no particular restriction on the mass ratio ((B)/(P)) of the content of Component (B) to the content of Component (C) defined by the phosphorus element concentration (P), on the basis of the total mass of the composition. However, the mass ratio ((B)/(P)) is preferably from 0.1 to 250, more preferably from 0.5 to 50, more preferably from 1 to 10, particularly preferably from 1.5 to 5.

The mass ratio of each of Components (A) and (B) to the content of Component (C) defined by the phosphorus element concentration (P) within the above-described ranges renders it possible to produce a composition which is excellent in anti-wear properties and initial anti-shudder properties and can easily retain the anti-shudder properties for a long period of time.

The use of Components (A), (B) and (C) in combination for the lubricating oil composition of the present invention renders it possible to produce a composition which is excellent in anti-wear properties and initial anti-shudder properties and can easily maintain the anti-shudder properties for a long period of time. However, in order to further enhance these properties or impart further properties necessary for a lubricating oil composition, conventional lubricating oil additives may be added to the lubricating oil composition of the present invention. Examples of such additives include metallic detergents other than Component (A), ashless dispersants, friction modifiers, anti-oxidants, extreme pressure additives, anti-wear agents, viscosity index improvers, rust inhibitors, corrosion inhibitors, pour point depressants, rubber swelling agents, anti-foaming agents, and dyes. These additives may be used alone or in combination.

Examples of metallic detergents other than Component (A) include salicylate detergents other than Component (A), sulfonate detergents, and phenate detergents.

Examples of salicylate detergents other than Component (A) include alkylsalicylic acid metal salts and/or (overbased) basic salts thereof, wherein the component ratio of the dialkylsalicylic acid metal salt having two alkyl groups having 10 to 40 carbon atoms is 15 percent by mole or more; dialkylsalicylic acid metal salts having an alkyl group having 1 to 9 carbon atoms and an alkyl group having 10 to 40 carbon atoms and/or (overbased) basic salts thereof; and alkylsalicylic acid metal salts and/or (overbased) basic metal salts thereof, wherein the component ratio of the 3-alkylsalicylic acid metal salt is less than 40 percent by mole. Examples of the metal include alkali metals and alkaline earth metals.

Examples of sulfonate detergents include alkali metal or alkaline earth metal salts, particularly preferably magnesium and/or calcium salts, of alkyl aromatic sulfonic acids, produced by sulfonating an alkyl aromatic compound having a molecular weight of 100 to 1,500, preferably 200 to 700. Specific examples of the alkyl aromatic sulfonic acids include petroleum sulfonic acids and synthetic sulfonic acids.

The petroleum sulfonic acids may be those produced by sulfonating an alkyl aromatic compound contained in the lubricant fraction of a mineral oil or may be mahogany acid by-produced upon production of white oil. The synthetic sulfonic acids may be those produced by sulfonating an alkyl benzene having a straight-chain or branched alkyl group, produced as a by-product from a plant for producing an alkyl benzene used as the raw material of a detergent or produced by alkylating polyolefin to benzene, or those produced by sulfonating dinonylnaphthalene. There is no particular restriction on the sulfonating agent used for sulfonating these alkyl aromatic compounds. The sulfonating agent may be a fuming sulfuric acid or sulfuric acid.

Specific examples of phenate detergents include alkaline earth metal salts, particularly magnesium salts and/or calcium salts, of an alkylphenolsulfide produced by reacting an alkylphenol having at least one straight-chain or branched alkyl group having 4 to 30, preferably 6 to 18 carbon atoms with sulfur or a Mannich reaction product of an alkylphenol produced by reacting such an alkylphenol with formaldehyde.

The base number of metallic detergents other than Component (A) is usually from 0 to 500 mgKOH/g, preferably from 20 to 450 mgKOH/g.

When a metallic detergent other than Component (A) is used, it is particularly preferably used in combination with one or more compounds selected from calcium sulfonate detergents and magnesium sulfonate detergents, each having a base number of 0 to 500 mgKOH/g, preferably 100 to 400 mgKOH/g. The use of these sulfonate detergents in combination renders it possible to produce a composition which has excellent and highly well-balanced anti-wear properties, anti-shudder properties and retainability thereof.

There is no particular restriction on the content of the metallic detergent other than Component (A) in the lubricating oil composition of the present invention. However, the content is usually from 0.01 to 5 percent by mass, preferably from 0.05 to 1 percent by mass, particularly preferably from 0.1 to 0.5 percent by mass, on the basis of the total mass of the composition.

The ashless dispersant which may be used in the present invention may be any compound that is used as an ashless dispersant for a lubricating oil. Examples of the ashless dispersant include nitrogen-containing compounds such as succinimides, benzylamines and polyamines, each having in their molecules at least one alkyl or alkenyl group having 40 to 400, preferably 60 to 350 carbon atoms, and derivatives or modified products thereof. The alkyl or alkenyl group having 40 to 400 carbon atoms may be straight-chain or branched and is preferably a branched alkyl or alkenyl group derived from an oligomer of an olefin such as propylene, 1-butene, or isobutylene, or a cooligomer of ethylene and propylene. An alkyl or alkenyl group having fewer than 40 carbon atoms would result in a compound with less dissolubility in a lubricating base oil while an alkyl or alkenyl group having more than 400 carbon atoms would cause the resulting composition to be poor in low-temperature fluidity.

Specific examples of the derivatives or modified products of nitrogen-containing compounds exemplified as an example of ashless dispersants include an acid-modified compound produced by allowing any of the above-described nitrogen-containing compounds to react with a monocarboxylic acid having 2 to 30 carbon atoms, such as fatty acid or a polycarboxylic acid having 2 to 30 carbon atoms, such as oxalic acid, phthalic acid, trimellitic acid, and pyromellitic acid, so as to neutralize or amidize the whole or part of the remaining amino and/or imino groups; a boron-modified compound produced by allowing any of the above-described nitrogen-containing compounds to react with boric acid so as to neutralize or amidize the whole or part of the remaining amino and/or imino groups; a phosphorus-modified compound produced by allowing any of the above-described nitrogen-containing compounds to react with phosphoric or phosphorous acid; a sulfur-modified compound produced by allowing any of the above-described nitrogen-containing compounds to react with a sulfuric compound; and modified products produced by a combination of two or more selected from the modifications with acid, boron, phosphorus and sulfur, of the above-described nitrogen-containing compounds.

The lubricating oil composition of the present invention may contain any one compound or two or more compounds, selected from the above-exemplified ashless dispersants in an any amount. However, the content is usually from 0.1 to 10 percent by mass, on the basis of the total mass of the composition.

The lubricating oil composition of the present invention contains preferably a boron-free ashless dispersant and/or a boron-containing ashless dispersant, which are preferably succinimide type ashless dispersants. The content of a boron-free ashless dispersant and/or a boron-containing ashless dispersant is preferably from 0.1 to 10 percent by mass, more preferably from 3 to 6 percent by mass, on the basis of the total mass of the lubricating oil composition. When these ashless dispersants are used in combination, the boron-free ashless dispersant is contained in an amount of preferably 10 percent by mass or less, more preferably 1 to 6 percent by mass, particularly preferably 2 to 4 percent by mass while the boron-containing ashless dispersant is contained in an amount of preferably 6 percent by mass or less, more preferably 0.01 to 4 percent by mass, particularly preferably 0.5 to 2 percent by mass. The mass ratio of the content of the boron-containing ashless dispersant to the total amount of the boron-containing ashless dispersant and the boron-free ashless dispersant is preferably from 0.1 to 1, more preferably from 0.15 to 0.5, particularly preferably from 0.2 to 0.3. Inclusion of a boron-containing ashless dispersant can enhance anti-wear properties and renders it easy to produce a composition having anti-shudder properties and retainability thereof that are excellent in a well-balanced manner and shifting characteristics.

The friction modifier which may be used in the present invention may be any compound that is usually used as a friction modifier for a lubricating oil. Specific examples include amine-, imide-, amide-, and fatty acid-type friction modifiers, each having in their molecules at least one alkyl or alkenyl group having 6 to 30 carbon atoms, particularly a straight-chain alkyl or alkenyl group having 6 to 30 carbon atoms.

Examples of amine-type friction modifiers include those such as straight-chain or branched, preferably straight-chain aliphatic monoamines, alkanolamines, and aliphatic polyamines, each having 6 to 30 carbon atoms, and alkyleneoxide adducts of these aliphatic amines.

Examples of imide-type friction modifiers include succinimide-type friction modifiers such as mono and/or bis succinimides having one or two straight-chain or branched, preferably branched hydrocarbon group having 6 to 30, preferably 8 to 18 carbon atoms, and succinimide-modified compounds produced by allowing such succinimides to react with one or more compounds selected from boric acid, phosphoric acid, a carboxylic acid having 1 to 20 carbon atoms, and sulfur-containing compounds.

Examples of amide-type friction modifiers include fatty acid amide-type friction modifiers such as amides of straight-chain or branched, preferably straight-chain fatty acids having 7 to 31 carbon atoms and ammonia, aliphatic monoamines, or aliphatic polyamines.

Examples of fatty acid-type friction modifiers include straight-chain or branched, preferably straight-chain fatty acids, fatty acid esters of such fatty acids and aliphatic monohydric alcohols or aliphatic polyhydric alcohols, fatty acid metal salts such as alkaline earth metal salts of such fatty acids (magnesium and calcium salts) and zinc salts of such fatty acids.

In the present invention, the above-described imide-type friction modifiers, in particular succinimide-type friction modifiers not only can enhance the friction coefficient of a wet clutch and thus improve power transmission efficiency but also are effective in significantly improving the anti-shudder durability.

Among the above-described friction modifiers, aliphatic amine-type friction modifiers and fatty acid-type friction modifiers, particularly fatty acid metal salts are particularly preferably used because they can significantly improve initial anti-shudder properties. Particularly preferably, the lubricating oil composition of the present invention contains such an aliphatic amine-type friction modifier and/or a fatty acid-type friction modifier. When an aliphatic amine-type friction modifier is used in combination with a fatty acid-type friction modifier, there is no particular restriction on the mass ratio therebetween. However, the mass ratio is preferably from 1:5 to 5:1, more preferably from 1:3 to 3:1, particularly preferably from 1:2 to 2:1 because more excellent shifting characteristics can be attained.

The lubricating oil composition of the present invention may contain any one compound or two or more compounds selected from the above-exemplified friction modifiers in an any amount. However, the content is usually from 0.01 to 5.0 percent by mass, preferably from 0.03 to 3.0 percent by mass, on the basis of the total mass of the composition.

The anti-oxidant which may be used in the present invention may be any anti-oxidant that is usually used in a lubricating oil, such as phenolic or aminic compounds.

Specific examples of anti-oxidants include alkylphenols such as 2-6-di-tert-butyl-4-methylphenol; bisphenols such as methylene-4,4-bisphenol(2,6-di-tert-butyl-4-methylphenol); naphthylamines such as phenyl-α-naphthylamine; dialkyldiphenylamines; esters of (3,5-di-tert-butyl-4-hydroxyphenyl) fatty acid (propionic acid) with a monohydric or polyhydric alcohol such as methanol, octadecanol, 1,6-hexanediol, neopentyl glycol, thiodiethylene glycol, triethylene glycol and pentaerythritol; phenothiazines; organic metal anti-oxidants such as molybdenum, copper, and zinc; and mixtures thereof.

One or more of these compounds may be blended in any amount in the lubricating oil composition of the present invention. However, the content of the anti-oxidants is usually from 0.01 to 5.0 percent by mass, on the basis of the total amount of the composition.

In the present invention, it is preferred to use a phenolic anti-oxidant and/or an aminic anti-oxidant and particularly preferred to use a phenolic anti-oxidant and an aminic anti-oxidant in combination because anti-shudder properties can be easily retain for a long period of time. When a phenolic anti-oxidant is used in combination with an aminic anti-oxidant, the mass ratio therebetween is preferably from 1:5 to 10:1, more preferably from 1:1 to 8:1, more preferably from 2:1 to 6:1.

The extreme pressure additive which may be used in the present invention may be any compound that is used as an extreme pressure additive for a lubricating oil. Examples of the extreme pressure additive include sulfuric compounds such as dithiocarbamates, disulfides, sulfurized olefins, and sulfurized fats and oils. One or more of these compounds may be blended in any amount in the lubricating oil composition of the present invention. However, the content is usually from 0.01 to 5.0 percent by mass, on the basis of the total mass of the composition.

Specific examples of viscosity index improvers include non-dispersant type viscosity index improvers such as polymers or copolymers of one or more monomers selected from various methacrylic acid esters or hydrogenated compounds thereof; and dispersant type viscosity index improvers such as copolymers of various methacrylic acid esters further containing nitrogen compounds. Specific examples of other viscosity index improvers include non-dispersant- or dispersant-type ethylene-α-olefin copolymers of which the α-olefin may be propylene, 1-butene, or 1-pentene, or a hydrogenated compound thereof; polyisobutylenes or hydrogenated compounds thereof; styrene-diene hydrogenated copolymers; styrene-maleic anhydride ester copolymers; and polyalkylstyrenes.

It is necessary to select the molecular weight of these viscosity index improvers, taking account of the shear stability thereof. Specifically, the number-average molecular weight of the non-dispersant or dispersant type polymethacrylate is from 5,000 to 150,000, preferably from 5,000 to 35,000. The number-average molecular weight of polyisobutylenes or hydrogenated compounds thereof is from 800 to 5,000, preferably from 1,000 to 4,000. The number-average molecular weight of ethylene-α-olefin copolymers or hydrogenated compounds thereof is from 800 to 150,000, preferably from 3,000 to 12,000.

Among these viscosity index improvers, the use of ethylene-α-olefin copolymers or hydrogenated compounds thereof renders it possible to produce a lubricating oil composition which is particularly excellent in shear stability.

One or more compounds selected from these viscosity index improvers may be blended in any amount in the lubricating oil composition of the present invention. However, the content of the viscosity index improver is usually from 0.1 to 20.0 percent by mass, on the basis of the total amount of the composition.

Examples of rust inhibitors include alkenyl succinic acids, alkenyl succinic acid esters, polyhydric alcohol esters, petroleum sulfonates, and dinonylnaphthalene sulfonates.

Examples of corrosion inhibitors include benzotriazole-, tolyltriazole-, and imidazole-type compounds.

Examples of pour point depressants include polymethacrylate conforming with a lubricating base oil to be used.

Examples of rubber swelling agents include aromatic- or ester-type rubber swelling agents.

Examples of anti-foaming agents include silicones such as dimethylsilicone and fluorosilicone.

Although the contents of these additives are optional, the content of the corrosion inhibitor is from 0.005 to 0.2 percent by mass, the content of anti-foaming agent is from 0.0005 to 0.01 percent by mass, and the content of the other additives is from 0.005 to 10 percent by mass, on the basis of the total amount of the lubricating oil composition of the present invention.

The kinematic viscosity at 100° C. of the lubricating oil composition of the present invention is usually from 2 to 25 mm²/s, preferably from 3 to 8 mm²/s, more preferably from 4 to 7 mm²/s, more preferably from 5 to 6 mm²/s.

The lubricating oil composition of the present invention is a lubricating oil composition which has excellent anti-wear properties for metal pulleys and metal belts and initial anti-shudder properties and can retain the anti-shudder properties for a long period of time, thus suitable for a continuously variable transmission with a slip-controlled wet clutch and a metal belt, particularly a wet starting clutch. Further, the lubricating oil composition is suitably used as a lubricating oil that is required to be improved in anti-wear properties and improve the characteristics of a wet clutch, specifically used for the automatic or manual transmission and the differential gears, of automobiles, construction machines and agricultural machines. Moreover, the lubricating oil composition can be used as gear oils for industrial uses; lubricating oils for the gasoline engines, diesel engines or gas engines of automobiles such as two- and four-wheeled vehicles, power generators, and ships; turbine oils; and compressor oils.

APPLICABILITY IN THE INDUSTRY

Monoalkyl-type salicylate detergents are excellent in initial anti-shudder properties but inhibit the anti-wear effect of a phosphorus-containing additive and fail to retain anti-shudder properties for a slip-controlled wet clutch. However, the combination of Component (A) with Components (B) and (C) according to the present invention renders it possible to inhibit wear in a continuously variable transmission wherein the metal pulleys and metal belt contact each other under severe conditions and prevent shudder peculiar to a slip-controlled wet clutch.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more details by way of the following examples and comparative examples, which should not be construed as limiting the scope of the invention.

EXAMPLES 1 TO 6 AND COMPARATIVE EXAMPLES 1 to 3

Lubricating oil compositions of Examples 1 to 6 and Comparative Examples 1 to 3 set forth in Table 1 were prepared to evaluate (1) initial anti-shudder properties, (2) retainability of the anti-shudder properties, and (3) anti-wear properties, as described below. The results are also set forth in table 1. The ratio of base oils was on the basis of the total base oil mass, and the amount of each additive was on the basis of the total mass of the composition.

(1) Initial Anti-Shudder Properties

Evaluation of initial anti-wear properties was carried out using LVFA at a surface pressure of 1.5 MPa in accordance with JASO M349-98 with regard to the other conditions so as to measure dμ/dV in a range of 2 rpm to 200 rpm. The dμ/dV value is an index that if it is positive and larger it indicates excellent initial anti-shudder properties while if it is negative it indicates that anti-shudder properties are extremely poor.

(2) Retainability of Anti-Shudder Properties

It was found that in a continuously variable transmission wherein the metal pulleys and metal belt contact each other under severe conditions, wear of metals and elution of iron ion occurred quite often and significant when Component (A) of the present invention was used and thus it was difficult for the resulting composition to retain anti-shudder properties. Each of the lubricating oil compositions was deteriorated at 150° C. for 72 hours in accordance with JIS-K2514 and then 300 ppm by mass of organic iron containing iron ion were dissolved in each composition in order to prepare lubricating oils having been used for a long period of time to be evaluated. Each oil was evaluated by measuring the dμ/dV in the same manner as (1). A positive and larger dμ/dV value indicates that the composition can retain anti-shudder properties for a wet clutch even though irons of the metal pulleys and metal belt elute into the lubricating oil due to the long time use.

(3) Anti-Wear Properties

A four ball wear scar diameter resulting from the use of each composition was measured in accordance with ASTM-D4172. The smaller the scar diameter, the composition is more excellent in anti-wear properties.

As apparent from the results set forth in Table 1, each of the compositions of Examples 1 to 6 indicated a positive and larger dμ/dV value that is a reference of initial anti-shudder properties and was remarkable in the retainability thereof. It is also confirmed that these compositions formed a scar the diameter of which was 0.50 mm or smaller and were also excellent in anti-wear properties. On the other hand, the compositions (Comparative Examples 1 to 3) not containing any of Components (A) to (C) are significantly poor in initial anti-shudder properties or anti-wear properties.

TABLE 1 Com- par- Compar- Compar- ative Exam- Exam- Exam- ative ative Exam- Example 1 Example 2 Example 3 ple 4 ple 5 ple 6 Example 1 Example 2 ple 3 Lubricating base oil (on the basis of total mass of base oil, mass %) Mineral oil A¹⁾ 50 50 50 50 50 50 50 50 50 Mineral oil B²⁾ 50 50 50 50 50 50 50 50 50 Additives (on th basis of total mass of composition, mass %) (A) Salicylate detergent³⁾ 0.6 0.6 0.6 0.4 1.2 0.6 0.6 — 0.6 (B) Nitrogen compound⁴⁾ 0.05 0.10 0.05 0.05 0.05 0.05 — 0.05 0.05 (C) Phosphorus-containing anti-wear 0.18 0.18 0.18 0.18 0.18 — 0.18 0.18 — agent A⁵⁾ Phosphorus-containing anti-wear — — — — — 0.28 — — — agent B⁶⁾ Other Ashless dispersant A⁷⁾ 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 additives Ashless dispersant B⁸⁾ 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Metallic detergent A⁹⁾ 0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.21 Metallic detergent B¹⁰⁾ — — 0.80 — — — — — — Friction modifier A¹¹⁾ 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 Friction modifier B¹²⁾ 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 Anti-oxidant A¹³⁾ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Anti-oxidant B¹⁴⁾ 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Anti-foaming agent¹⁵⁾ 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 Amount of Ca originating from (A) mass % 0.04 0.04 0.04 0.03 0.08 0.04 0.04 0.00 0.04 Amount of P originating from (C) mass % 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.00 Initial anti-shudder properties (dμ/dV) 0.007 0.007 0.006 0.005 0.009 0.007 0.007 −0.003 — Anti-shudder properties after deterioration (dμ/dV) 0.004 0.005 0.005 0.002 0.006 0.004 −0.004 −0.006 — Anti-wear properties 0.45 0.43 0.47 0.43 0.49 0.45 0.65 0.43 0.65 (Four-ball test wear-scar diameter) mm ¹⁾Hydrocracked base oil (100° C. kinematic viscosity: 2.6 mm²/s, viscosity index: 105, NOACK evaporation loss: 52 mass %, S: 0.1 mass % or less) ²⁾Hydrocracked base oil (100° C. kinematic viscosity: 4.0 mm²/s, viscosity index: 125, NOACK evaporation loss: 16 mass %, S: 0.1 mass % or less) ³⁾Calcium carbonate overbased salt of alkylsalicylic acid calcium salt having C14 to C18 secondary alkyl group (Base number: 170 mg KOH/g, Ca: 6 mass %), Sructure of alkylsalicylic carbonate: 3-alkyl: 53 mol %, 4-alkyl: 4 mol %, 5-alkyl: 35 mol %, 3,5-dialkyl: 8 mol % ⁴⁾2.5-bis(alkyldithio)-1,3,4-thiaziazole, alkyl group: C6-C8 alkyl group ⁵⁾dibutylphosphite (phosphorus content: 16.4 mass %) ⁶⁾di-2-ethylhexylphosphite (phosphorus content: 10.6 mass %) ⁷⁾polybutenyl succinimide ⁸⁾boric acid-modified polybutenyl succinimide ⁹⁾calcium sulfonate (total base number: 300 mgKOH/g) ¹⁰⁾magnesium sulfonate (total base number: 100 mgKOH/g) ¹¹⁾amine-type friction modifier ¹²⁾fatty acid-type friction modifier ¹³⁾dialkyl diphenylamine ¹⁴⁾bisphenolic anti-oxidant ¹⁵⁾polydimethylsiloxane 

1. A lubricating oil composition suitable for a transmission with a slip-controlled wet clutch, comprising: a lubricating base oil; (A) an alkylsalicylic acid metal salt and/or an (overbased) basic salt thereof, wherein the component ratio of the monoalkylsalicylic acid metal salt is from 85 to 100 percent by mole, the component ratio of the dialkylsalicylic acid metal salt is from 0 to 15 percent by mole and the component ratio of the 3-alkylsalicylic acid metal salt is from 40 to 100 percent by mole; (B) a nitrogen compound represented by formula (1) below; and (C) a phosphorus-containing anti-wear agent:

wherein R¹ is a straight-chain or branched alkyl group having 1 to 30 carbon atoms, R² is hydrogen or a straight-chain or branched alkyl group having 1 to 24 carbon atoms, and a and b are each independently an integer of 1, 2 or
 3. 2. The lubricating oil composition according to claim 1, further comprising a friction modifier. 